1. Field of the Invention
The present invention relates generally to the construction of wells and more specifically to the handling and storage of pipe used in the drilling, completion and workover of wells. The primary applicability of the invention is for wells constructed through deep-water bodies of 3,000 ft. or more in depth. The invention also has utility in the construction of certain onshore wells and offshore wells drilled through shallower water bodies.
2. Background of the Prior Art
The pipe used in constructing wells is typically assembled with individual pipe sections, referred to as "joints", that are secured together, end-to-end, to form a pipe string. The different pipe strings used in the construction of the well include drill strings, work strings, casing strings, and tubing strings, as well as others.
The drill strings and work strings are employed in drilling and/or repairing the well. The drill strings and work strings may be run into and out of the well many times during the initial well construction or during the performance of subsequent well modifications or repair.
The casing and tubing strings are generally a permanent part of the well and are respectively used to maintain the well bore structure and to convey produced well fluids to surface production facilities. The casing is generally run into the well during the initial well construction and is not usually retrieved during the normal life of the well. The tubing string is usually run into the well once and remains in the well during normal operation of the well. In some cases, the tubing must be withdrawn from the well to repair or workover the well. The term "string" may be used to designate two or more connected joints of pipe.
The major equipment component used to drill a well is generally referred to as a drilling rig. In many cases, after the well is drilled, the drilling rig is removed and smaller rigs called completion or workover rigs are employed to complete the well construction. The smaller rigs are also used to workover wells that have been previously drilled by a larger drilling rig. For convenience, the terms "drilling rig" or "drilling platform" or "drilling structure", as employed herein in the description of the present invention, may be used to include drilling, workover and completion rigs and other analogous equipment such as hydraulic snubbers that may be used in the construction of a well. The term "drilling" may be used to include the procedures of drilling, completing and working over a well as well as other related aspects of the construction of a well. The term "drill string" may be used to include tubing strings, work strings, casing strings and other pipe strings that may be employed in the construction of a well.
The equipment used to construct wells varies from small land-based rigs to enormous deep-water rigs. The water based rigs include platform mounted rigs, jackup rigs, semisubmersible rigs, drill ships, tension leg platforms and various other types and kinds of structures. These drilling rigs have, in common, a drilling rig "floor" from which the drilling operations are primarily initiated and controlled. The water-based rigs also typically employ a string of riser pipe that extends between the rig and an underwater wellhead located on the water bottom. The riser pipe is used in the construction of the well and, in some cases, may be used during the production of the well.
The individual pipe joints used in the construction of a drill string are transported from a storage location and assembled end-to-end on the drilling rig. The assembled joints form a string that is lowered into the well. When the string is withdrawn from the well, the individual joints are removed from the string at the drilling rig floor and transported to a storage location.
A large part of the time required to construct a well is expended in assembling and disassembling the string and handling the pipe joints as they are moved between the storage location and the well. Personnel working on the rig floor using manually operated handling tools usually do the work of assembling and disassembling the string. The work is time consuming and can be hazardous.
The cost of a well is directly related to the time required to construct the well. The cost considerations become critically important in the drilling of deep-water wells because the deep water drilling rigs and the required support services are extremely expensive. Operating costs of $10,000 per hour, or more, may be incurred in deep- water drilling operations.
Deep-water drilling rigs employ specialized, complex equipment to store, assemble, and disassemble the pipe strings. Pipe withdrawn from the well is disassembled on the rig floor and may be stored on horizontal pipe racks located on the drilling structure. For convenience and timesavings, the pipe, such as drill pipe, that is run into and out of the well a number of times, is stored vertically in "stands" within the rig derrick. The stands are made up of three joints secured end-to-end and extend approximately 90 ft. up into the derrick.
Some newer deep-water rigs are equipped with tall derricks that handle stands of four joints. The addition of the extra joint in each stand reduces the number of times that the drill string must be disconnected and reconnected as it is withdrawn and rerun back into the well. The addition of the extra joint reduces the time required to run and retrieve the drill string and reduces the handling damage done to the string while connecting and disconnecting the joints.
It is apparent that even greater benefits could be achieved if the length of the stands could be increased beyond the current limit of four joints. However, using conventional techniques, the derrick height places a practical limit on the length of the stands. The maximum stand length for a stand of four joints of drill pipe is typically less than 125 ft.
Using coiled pipe can offset some of the disadvantages of using jointed pipe to work in wells. Coiled pipe is a continuous, non-jointed string of steel pipe that is stored on large reels situated on the rig structure. Coiled pipe eliminates the need to make and break the connections between individual joints of a pipe string and permits continuous pipe injection or extraction. The driving system employed to inject the coiled pipe into the well grips the pipe, straightens it, and continuously feeds it from the reel into the well. The driving system re-bends the pipe and coils it back onto the reel as the pipe is extracted from the well. Coiled pipe can be run and extracted much faster than conventional jointed pipe.
While coiled pipe may be quickly injected into and retrieved from a well, it has limited use in well construction. Coiled pipe suffers from the disadvantage of not being rotatable within the well. Moreover, the requirement to repeatedly straighten and re-bend the pipe beyond its non-yielding curvature limits the size and strength of the pipe. This limitation, in turn, restricts the pipe's ability to withstand forces normally encountered in the drilling and completion of wells, particularly deep, high-pressure wells.
The prior art has suggested storing a column of conventional jointed drill pipe or tubing in a continuous piece without breaking it into shorter sections. U.S. Pat. No. 3,724,567 to Smitherman teaches moving a pipe column in an upwardly and radially outwardly curved path from the well bore to a substantially horizontal attitude for storage preferably in a generally circular configuration on the ground or support means for later return to the well bore moving approximately along the same path, and as one piece. Smitherman discloses a pipe elevator system that can continuously remove the drill string from the well bore. A support and a guide are used to redirect the pipe from the vertical to the storage location and position.
The Smitherman apparatus, when employed in an offshore installation, is described as supporting the stored drill pipe on the drilling barge in a rotatable, circular storage rack. The drill string is bent as it is redirected from the derrick down to the circular storage rack and then bent again as it is coiled into the storage rack. The bending radius of the pipe is sufficiently large to prevent yielding so that the pipe will return to a straight posture once the bending forces are removed. The diameter of the storage rack must be at least twice the length of the permissible bending radius with the result that the rack requires significant space on the drilling barge.
U.S. Pat. No. 5,547,314 to Ames describes a system and method for storing a continuous link of an assembled jointed tubular conduit on an offshore vessel and for conveying the tubular conduit between a deck of a vessel and a subsurface well head on the floor of the body of water. The vessel carries a carousel positioned around its perimeter. The assembled tubular conduit is maintained in coiled storage within the carousel. Horizontal and vertical directing means are provided that, in cooperation with rotation of the carousel, enable uncoiling of the tubular conduit from the carousel into the well or recoiling of the tubular conduit into the carousel from the well.
Like the Smitherman apparatus, the Ames system and method store the well tubular on the offshore vessel in a rotatable carousel. The Ames carousel encircles the perimeter of the vessel. To prevent yielding the jointed drill pipe, the radius of the carousel is maintained greater than that of the smallest bending radius of the pipe. Also, as the with the Smitherman apparatus, the drill pipe is bent as it leaves the derrick and is redirected toward the carousel and then bent again as it is directed into the carousel.
U.S. Pat. No. 4,848,455 to Fenyvesi describes an apparatus for deep drilling in which the pipe sections are connected together with special flexible connections permitting the pipe string to be stored on the surface in a circular storage casing smaller than that possible with the rigid pipe connections of the Smitherman or Ames designs. Fenyvesi describes a pipe connection that permits bending between the connected joints when the pipe is to be stored. The connection can be locked to prevent such bending when the pipe is inserted into the well.
Relatively complex, expensive equipment is required to provide a carousel and to effect the necessary compound bending and carousel rotation that are part of the prior art storage designs. The requirement for special flexible joints in the drilling string, while reducing the space requirements of the carousel, adds additional complexity and cost to the drilling operation.
Each of the described prior art pipe storage designs occupies a large area on the vessel and requires that the vessel support the weight of the stored pipe.